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Height

Hi Chaps,

I use a slim jim on the top of a few pieces of wood which I use as a “mast” when I activate (yes, really!). I always try and raise the antenna up as far as possible and my desire for height has resulted in the poles breaking on several occasions!

I’m having an interesting conversation with one of my ham mates who says that because we’re already up so high on a summit, an extra 5 metres on top of an already 5 metre pole would make little difference.

Any thoughts!?

Thanks,

Rob G7LAS

In reply to G7LAS:

I’m having an interesting conversation with one of my ham mates who says that
because we’re already up so high on a summit, an extra 5 metres on top of an
already 5 metre pole would make little difference.

They’re essentially wrong. The question to consider is how any reflected waves from the ground interfere with the directly radiated wave from the antenna. There will be certain heights for certain shaped ground profiles were the height will make a lot of difference.

Andy
MM0FMF

In reply to G7LAS:

I’m having an interesting conversation with one of my ham mates who
says that because we’re already up so high on a summit, an extra 5
metres on top of an already 5 metre pole would make little difference.

My guess is that the extra 5m would not help very much. However, it may depend on the shape of the summit, and your exact location.
I remember an article in Radcom (quite a lot of years ago so I cannot quote the reference), of some tests on (I think) one of the Malvern hills. This suggested that some “gain” could be achieved from signals reflected off the slope and adding to those directly arriving at the antenna. The effect depends upon the angle of the slope, and the relative location of the antenna. I think there was a similar suggestion regarding HF antennas on slopes, in a different article…

Anyway, if you are on a plateau some way back from the top of a slope, then increasing the height of your antenna might be an advantage.

Forty odd years ago, we used to enter VHF NFD from the Brown Clee, and the 2m station was located at the top of a slope on the SE side of the hill. We did notice that best DX seemed to cluster in a fairly narrow angle around the direction of the slope, but no experiments were performed to establish a cause and effect.

I think some experiments are called for, and a report back!

Adrian
G4AZS

In reply to G7LAS:

I’m having an interesting conversation with one of my ham mates who
says that because we’re already up so high on a summit, an extra 5
metres on top of an already 5 metre pole would make little difference.

Any thoughts!?

Hi Rob,

I take a minimum of 2 x 5’ poles and nearly always 3, giving the white stick a clearance of 15’ from the ground.

Of course, these have to mauled up to the summit :frowning:

73 Mike
2E0YYY

In reply to MM0FMF:

A complex question! For a start it will depend on the band.

One factor to remember is that the waves don’t reflect from the ground so much as a relatively conductive layer where the soil is saturated. On a vegetated summit that will probably be close to ground level, but on a rocky summit it will probably correspond to the depth in the rock where the inevitable fractures (due to freeze-thaw in the ice age and load relief from erosion) are holding water. On a summit like, say, Tryfan the reflections might come from many metres below ground level.

73

Brian G8ADD

Look for Software like HFTA or its old predecessor YTAD (don´t know whether YTAD will still run on many modern machines). You insert the topografic numbers of the slope to genrate its profile (a bit of a chore…) and can then play with the type of antenna, its height above the summit and its distance from the slope. The software will show you the resulting elevation angles for each choosen frequency.

With this you can assume whether it is necessary to use more height or whether 5m are enough (earth losses are another topic).
In general you can expect more power to the low elevation angles the higher the antenna is, even on a summit with a steep slope. As “antenna heigth” is height in fractions of wave length more height in meters will especially be helpful on 40/30m - as long as you want low elevation angles.
For low elevation angles an early steep slope below the summit is more helpful than a not so steep slope with many meters of height difference. “Steps” with horizontal parts in the slope don´t help either.

You can compare several antennas or heights or configurations of one antenna at the same run of the program which then generates overlapping lines in one diagram. You can see output diagrams here: http://www.dl8mbs.de/40993/44860.html
It is fascinating to play with this program and it gives really good insights. But it works for horizontally polarized antennas onlys. For vertical polarized antennas there was a similar software called TA by K6STI which I never could get (it is said to be DOS-software). Only by anecdotal eveidence I heard that have verticals like GPs don´t like to loose the ground under their initial wave (i.e. steep slopes). Complete verticals like quads need less heigth than dipoles but can nevertheless gain with height.

The shape of the slope (convex vs. concave) can not be simulated - Moxon wrote that grooves/gullies (excuse my english) in the desired direction were more helpful than the opposite.

So I only need help to make it run again on W7 on my PC which others were able to…

edit: of course no scientific claim :wink: :wink:

In reply to G7LAS:

I wrote an article about this subject in Radcom about 5 years ago.

73

Richard
G3CWI

In reply to G7LAS:
Hi Rob,

I too have recently started to use a dual band ladder line slim jim on top of a 10m Linfaee roach pole.

Based on my own experiences, since changing over to this for 2m fm and 70cms fm,I can only say yes it does make a difference.

I’m not the technically minded type so I’ll leave that to others when it comes to radio and aerials. All I can say is that it’s working for me with some very intersting and favourable results.

Little Mell Fell is one good example. From my home QTH this is not workable, even with a lift on. The first time I used this set up on this fell, Colin almost managed to work me from the home QTH.

The same also for the SOTA/WOTA summit Halin Fell. Another difficult fell to work from due to it’s location.

I have also been getting much more favourable reports from chasers on fells worked before and in the case of the fell above, even managed to work into Scotland on 2m fm for a S2S contact with report of 5/9.

This weekend Colin and I worked another difficult fell to get out from, the WOTA summit of Barf, yet we managed to get 12 contacts and worked into Annan in Scotland at 5/9 also. We also managed to get Neil 2M0NCM from our third summit of the day WOTA summit Broom Fell, bringing a total of seven S2S’s for the day. Both of these were much lower than Lord’s Seat and Barf in particular is quite enclosed.

I seriously doubt we would have has this level of success without the additional height.

73

Liz

In reply to G4AZS:
This suggested that some “gain” could be achieved

from signals reflected off the slope and adding to those directly
arriving at the antenna. The effect depends upon the angle of the
slope, and the relative location of the antenna. I think there was a
similar suggestion regarding HF antennas on slopes, in a different
article…

EMEers reckon to get about 6dB of gain when the moon is at the right
angle above the horizon.

73

Rick

THE right angle, or A right-angle?

In reply to M1EYP:

“The”

Andy
MM0FMF

In reply to M0RCP:

This article explains the concept of ground gain:

http://www.oz1rh.com/

73

Rick

In reply to M0RCP:

In reply to M0RCP:

This article explains the concept of ground gain:

http://www.oz1rh.com/

Interesting article, thank you! It does mention that ground gain is more significant with horizontal polarisation - so the effect on a vertical J Pole as described at the start of this topic is likely to be marginal.

That is not to contradict Liz’s observations - additional height on specific summits could be an advantage for other reasons…

Adrian
G4AZS

I generally use an endfed 1/2 wave antenna for 2m mounted directly on the radio. I generally use it that way on the FT-817. I have found that often chasers report I have a better signal if I hold the radio above my head rather than at shoulder height. I think the difference is 2 or 3 dB not 5 or 10 dB, but in some cases that is the difference between making a contact or not. It does make my arm tired fairly quickly.

No matter where an antenna is located it will work better if it is up in the clear away from local obstructions. So at 2.5m it is better than at 2m. At some height the advantage of even more height will start to diminish. I am not sure at what height that point is reached and of course it will depend upon which frequency band is being used.

Some comments about radio waves reflecting off of the ground. At lower frequencies the radio waves may penetrate the ground until they reach a conductive layer as was described above. Radio waves will also reflect off of dielectric and/or magnetic materials. This effect is more more noticeable at higher frequencies. At VHF and above I would expect most of the energy to be reflected from the surface of the ground even if the ground is not very conductive at the surface.

Doug, N7NGO

In reply to G4AZS:

Hi Adrian,

No problem. As I said, Im not a tech buff so much of this is way over my head.

As I have found in the Lake District Area, you get some very strange and unpredictable results at times when in theory the contact should not have been achievable.

As I have also learnt from the early days of Colin operating with just a handheld before obtaining my own licence, even the added height of the handheld held aloft on the end of a walking pole can be enough to allow the contact far more easily or can be the difference in getting the contact at all. The same applies to which way you turn/stand when using just a handheld.

When operating in the latter method, I now make a point of turning 360 degrees to see in which direction the signal is strongest. At times it makes a big difference on the incoming siganl.

Hopefully, once the better weather starts, Colin and I can operate some fells togther a bit more often to get a much better direct comparison. On the occasions when we have done so, there have been times when we know we would not have suceeded on just a hand held aerial as we both carry our own radios and whilst one 2m is connected to pole, the other is left as handheld either scanning the bands or listening on the current contact where possible.

I wonder if some other factors are the type of ground operated from. When working the fells above Honister, there were some areas where the compass would just spin and even the GPS was not particularly happy. Standinng on some of these rocks made for interesting results. Sometimes a couple of steps away from one point would totally loose the signal.

All adds to the fun

73’

Liz

In reply to M6EPW:

much of this is way over my head

It will be if it’s on a 10m mast! :slight_smile:

Sorry, I’ll be serious now. You’re right about turning around when using a handheld and its rubber duck etc. The human body is a big lossy lump of lard and salty water as far as the RF is concerned. Ensuring the lossy lump is not between the aerial and the DX will often make a huge difference. As will elevating the antenna above head height.

As equinox has past and the WX now starts to improve rapidly (ha!) you can experiment with the antennas you have. Play with the absolute height by not pushing up the mast as far or by standing nearer the sloping ground. If you note the antenna arrangement in your logs you’ll be able to see what worked well on previous summits with similar terrain. You don’t have to be a tech buff to make and apply these observations.

Andy
MM0FMF